: A functional assay system for monitoring antithrombin drugs, such as recombinant hirudin and hirudin analogs, in patients will be developed. The proposed assay system will be based on a novel dry reagent paramagnetic particle-based assay technology that has recently been applied successfully to whole blood, bedside monitoring of prothrombin time and activated partial thromboplastin time testing. The dry reagent assay consists of paramagnetic iron oxide particles whose oscillations in an imposed magnetic field will be dampened by the formation of a fibrin clot around the particles. The clotting time in turn is related to the concentration of thrombin inhibitor present in the blood sample. Specific aims include: (1) compare the aPPT-based assay with a thrombin- based test system; (2) refine the prototype aPPT-based hirudin assay or thrombin time-based assay by test card optimization and testing; (3) develop algorithms for rapid calibration and standardization to a reference method; and (4) study the effects of potential influencing factors on the assay and determine performance characteristics. Introduction of a rapid, convenient assay system for antithrombin drugs at the patient point-of-care could provide results earlier than existing and newly-emerging laboratory methods and could serve as an important diagnostic guide during therapy.